ddev import-db should accept data from /dev/stdin

[ghost]

Is your feature request related to a problem? Please describe.
I have my SQL data stored in several files. I would like to import them to a DB.

Describe the solution you'd like
It would be nice to be able to pipe SQL data in ddev --import-db like

cat db/sql/struct/* db/sql/data/* | ddev --import-db

Describe alternatives you've considered

I use

cat db/sql/struct/* db/sql/data/* | docker exec -i ddev-<PROJECTNAME>-db mysql --default-character-set=utf8 db

as a workaround to pipe SQL data into the "mysql" command in the container.
Notice the docker exec -i which connects the host's stdin to the command.

[rfay]

Thanks!

[rfay]

Actually, #1700 gets us set up for this, but doesn't actually solve this problem, reopening.

[rfay]

The example in ddev import-db -h doesn't have any information about this usage, still needs to be added. Also we should check the docs.